1. Field of the Invention
The present invention relates to a positive electrode active material for a non-aqueous electrolyte-based secondary battery and a non-aqueous electrolyte-based secondary battery using the same, and more specifically relates to a positive electrode active material for a non-aqueous electrolyte-based secondary battery, composed of a lithium/nickel composite oxide with high capacity, low cost and excellent heat stability, and a non-aqueous electrolyte-based secondary battery, having high capacity and high safety, using the same.
2. Description of the Prior Art
Recently, with rapid expansion of a compact sized electronics device such as a mobile phone and a notebook-type personal computer, demand of a non-aqueous electrolyte-based secondary battery has rapidly growing, as a power source which is capable of charging and discharging. As a positive electrode active material for a non-aqueous electrolyte-based secondary battery, in addition to a lithium/cobalt composite oxide represented by lithium cobaltate (LiCoO2), a lithium/nickel composite oxide represented by lithium nickelate (LiNiO2), a lithium/manganese composite oxide represented by lithium manganate (LiMnO2) and the like are also widely used.
Note that lithium cobaltate had a problem of containing cobalt, as a main component, which is expensive due to scarce amount of reserves, and thus unstable in supply and also large cost fluctuation. Therefore, a lithium/nickel composite oxide or a lithium/manganese composite oxide is noticed in view of cost, because of having relatively low cost nickel or manganese, as a main component. Lithium manganate, however, has many practical problems as a battery, because capacity is very small as compared with other material, and charge and discharge cycle characteristics showing a life time, is also very short, although having excellent heat stability as compared with lithium cobaltate. On the other hand, lithium nickelate is expected as a positive electrode active material which is capable of producing a battery with high energy density in low cost, due to showing larger capacity than lithium cobaltate.
Lithium nickelate, however, had a defect of poor heat stability in charged state than lithium cobaltate, in any of the following powder shapes; lithium nickelate is usually produced by mixing a lithium compound and a nickel compound such as nickel hydroxide or nickel oxyhydroxide, and the firing, and has a powder shape of primary particles with single dispersion, or powder shape of secondary particles having voids, which is an aggregation of primary particles. Namely, pure lithium nickelate had problems of safety like heat stability and the like, charge and discharge cycle characteristics and the like, which had inhibited use as a practical battery; this is because of lower stability of crystal structure in charged state as compared with lithium cobaltate.
As a solution, it is general to obtain a lithium/nickel composite oxide which has good safety, and charge and discharge cycle characteristics, as a positive electrode active material, by substitution of a part of nickel with a transition metal element such as cobalt, manganese and iron, or a heterogeneous element such as aluminum, vanadium and tin, to stabilize crystal structure in a state of lithium being eliminated by charging (see “High density lithium secondary battery”, Technosystem Inc., Mar. 14, 1998, pages 61 to 78 or JP-B-3244314 (p. 1 and p. 2)). In this connection, however, small amount of the element substitution cannot attain sufficient improvement of heat stability, while much amount the element substitution causes capacity reduction, and thus superiority as material of a lithium/nickel composite oxide could not be well utilized in a battery.
In addition, to reduce reactivity of an electrolysis solution with a positive electrode active material in a battery, a method for enhancing heat stability by reducing specific surface area of a positive electrode active material, which area is used as an index to reduce reaction surface area thereof, has been proposed (see, for example, JP-A-11-135123 (p. 1 and p. 2)). This method, however, only found out that simultaneous introduction of a small amount of aluminum and yttrium to a lithium composite oxide improves heat stability and reduces specific surface area of the lithium composite oxide, and thus suppresses reactivity between positive electrode material and an electrolysis solution, in overcharge; here, because specific surface area in an adhered state of impurities or by-products at the particle surface of a positive electrode active material is noticed as for reaction surface area of a positive electrode active material with the electrolysis solution, specific surface area used here does not represent true reaction surface area.
Note that, as a modification method for a positive electrode active material, a method for removing impurities or by-products adhered at the positive electrode active material, by water washing has been proposed (see, for example, JP-A-9-231963 (p. 1 and p. 2), JP-A-9-259879 (p. 1 and p. 2) or JP-A-2003-17054 (p. 1 and p. 2)).
Washing of a lithium/nickel composite oxide randomly with water, however, generated the following defects. First of all, proposals of JP-A-9-231963 (p. 1 and p. 2), JP-A-9-259879 (p. 1 and p. 2) or JP-A-2003-17054 (p. 1 and p. 2) aim at improvement of charge and discharge cycle characteristics, stabilization of electrode property and gas generation in charging, and thus do not notice reaction surface area between a positive electrode active material itself and an electrolysis solution. Therefore, there is no description on the effect of change in specific surface area of powders after water washing, and improvement of heat stability. In addition, the washing technology with water had a problem of much amount of elution of lithium ions, or generation of structure change caused by high temperature, namely change in a substance itself, in the limited case of a lithium/nickel composite oxide, because of low slurry concentration or carrying out heat processing at high temperature after water washing.
Namely, a lithium/nickel composite oxide obtainable by a usual production method is a substance which is more difficult to reduce specific surface area by a method providing industrially high productivity while maintaining good electrochemical characteristics, compared with a lithium/cobalt composite oxide. Furthermore, even when specific surface area of the lithium/nickel composite oxide obtained after firing could be reduced, a problem of practically not improving heat stability of the positive electrode active material frequently occurred. Namely, in the case where specific surface area after firing is used as an index, which has conventionally been used as an index of reaction surface area, a lithium/nickel composite oxide with high heat stability could not industrially be produced in a stable manner.
From the above situation, further technical improvement such as improvement method for heat stability and new index thereof or the like has been required, by obtaining a positive electrode active material for a non-aqueous electrolyte-based secondary battery from a lithium/nickel composite oxide having high capacity, low cost and excellent heat stability, and so as to attain a low cost battery with high energy density, using the same.